The present invention relates to a force detector using capacitance elements or variable resistance elements and, more specifically, to a force detector suitable for being applied to an input device which obtains operational inputs indicating predetermined operation amounts into an electronic apparatus such as a portable telephone, game machine, or the like which executes predetermined processing based on predetermined programs.
In an electronic apparatus such as a portable telephone, game machine, or the like, a predetermined operational input by a user is accepted, and based on this operational input, an execution of program proceeds. Normally, such an operational input is accepted while a user looks at a cursor or other objects displayed on a display screen, and an input indicating one of four directions including upward, downward, rightward, and leftward directions, or one of eight directions including diagonal directions as well are generally required. A device called a joystick has been used for executing such an input with directivity. Such type of device normally includes a built-in two-dimensional force detector which detects a direction and an operation amount of the applied operational input by independently detecting a component in the X-axis direction and a component in the Y-axis direction. For example, an operational input with a +5 component in the X-axis direction indicates an operation amount of 5 in the rightward direction, and an operational input with a −8 component in the Y-axis direction indicates an operation amount of 8 in the downward direction. Of course, detection of an operational input applied diagonally is also possible by carrying out an operation for composing the component in the X-axis direction and the component in the Y-axis direction, and so forth.
In an electronic apparatus such as a portable telephone, game machine, or the like, a clicking input is required as well as the abovementioned operational input with directivity. This clicking input is basically an input indicating a binary condition of ON/OFF, and it is important to provide an operator with a clicking sensation through this input. Therefore, a reaction force must be applied with respect to a pressing force applied from the operator's finger while securing some degree of stroke. As a switch suitable for such ON/OFF inputs with a clicking sensation, switches using elasticity of elastic materials such as rubber and metals have been generally used, and force detectors with a function for making operational inputs in predetermined directions as well as clicking inputs have been made practicable.
As relatively inexpensive input devices for electronic apparatuses, force detectors using capacitance elements have been frequently used. For a force detector using a capacitance element, a structure in which the distance between a couple of electrodes is changed by externally applied forces is employed, and by electrically detecting this electrode distance as a capacitance value for the capacitance element, the externally applied forces can be determined. Thus, basic components of a force detector using a capacitance element are a pair of electrodes, wherein advantages of a simple structure and low parts costs can be obtained. Therefore, a force detector using a capacitance element has been widely used as an input device for an electronic apparatus such as a portable telephone, game machine, or the like.
As a method for electrically detecting a capacitance value C of a capacitance element, a method using a C/V converter circuit for converting the capacitance value C into a voltage value V and a method using a C/f converter circuit for converting a capacitance value C into a frequency f have been generally known. However, general C/V converter circuits and C/f converter circuits have oscillation circuits inside, so that power consumption during operation is relatively great. Therefore, if conventional force detectors using such capacitance elements are installed into various electronic apparatuses, power consumption increases as a whole. Particularly, a design in which battery consumption is reduced as long as possible has been demanded for electronic apparatuses such as a portable telephone, game machine, or the like which are operated by built-in batteries. Therefore, a force detector using a capacitance element is disadvantageous in terms of power consumption although it is advantageous in terms of manufacturing costs.
Of course, a measure for reducing the entire power consumption by intermittently operating a C/V converter circuit or C/f converter circuit with great power consumption can be taken. For example, if an intermittent operation with 200 msec periods is carried out in which the circuit is operated for 20 msec and then stopped for the next 180 msec, five measurements per second are possible while the power consumption is reduced to be one tenth. However, even by taking this measure, wasteful power consumption cannot be completely suppressed. Considering the actual use pattern of a portable telephone or the like, the period of input operation for cursor movements or the like is very short, so that operation of circuits with great power consumption while an operator does not carry out operational inputs at all is not efficient.
Also, in Japanese Patent Application No. 2000-132012, a force detector using a variable resistance element is proposed. In this detector, a variable resistance element whose resistance value changes in accordance with applied pressures is used, and externally applied forces can be detected by detecting changes in resistance value of this variable resistance element.
In this force detector using the variable resistance element, to obtain a detection value of an applied force, a measurement of electric resistance of a resistor is essential. However, to measure the electric resistance of the resistor, it is necessary to make a current flow in the resistor, and some degree of power consumption is inevitable during the measurement. Therefore, if the abovementioned force detector using the variable resistance element is installed into various electronic apparatuses, power consumption increases as a whole.